Higher operating temperatures for gas turbine engines are continuously being sought in order to improve their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel, and cobalt-based superalloys. Still, with many hot gas path components constructed from supper alloys, thermal barrier coatings (TBCs) can be utilized to insulate the components and can sustain an appreciable temperature difference between the load-bearing alloys and the coating surface, thus limiting the thermal exposure of the structural component.
While superalloys have found wide use for components used throughout gas turbine engines, and especially in the higher temperature sections, alternative lighter-weight substrate materials have been proposed, such as ceramic matrix composite (CMC) materials. CMC and monolithic ceramic components can be coated with environmental barrier coatings (EBCs) to protect them from the harsh environment of high temperature engine sections. EBCs can provide a dense, hermetic seal against the corrosive gases in the hot combustion environment.
Silicon carbide and silicon nitride ceramics undergo oxidation in dry, high temperature environments. This oxidation produces a passive, silicon oxide scale on the surface of the material. In moist, high temperature environments containing water vapor, such as a turbine engine, both oxidation and recession occurs due to the formation of a passive silicon oxide scale and subsequent conversion of the silicon oxide to gaseous silicon hydroxide, which results in dimensional loss of the material. For component applications of silicon-based substrates in turbine engines, such material loss can open up clearances and may lead to efficiency losses, and ultimately may lead to perforation of the component.
As such, an environmental barrier coating (EBC) is applied onto the surface of the ceramics to help protect the underlying component. Current materials commonly used for environmental barrier coatings on CMC's include celsian-phase barium strontium aluminosilicate (BSAS) and rare earth silicates. All of these materials are relatively stable in steam compared to the CMC and can prevent penetration of steam to the CMC if present as a dense coating layer.
However, these materials have varying resistance against molten environmental contaminant compositions, particularly those containing oxides of calcium, magnesium, aluminum, silicon, and mixtures thereof. Dirt, ash, and dust ingested by gas turbine engines, for instance, are often made up of such compounds, which often combine to form contaminant compositions comprising mixed calcium-magnesium-aluminum-silicon-oxide systems (Ca—Mg—Al—Si—O), hereafter referred to as “CMAS.” At the high turbine operating temperatures, these environmental contaminants can adhere to the hot barrier coating surface, and thus cause damage to the EBC. For example, CMAS can form compositions that are liquid or molten at the operating temperatures of the turbines. The molten CMAS composition can dissolve the barrier coating, or can fill its porous structure by infiltrating the pores, channels, cracks, or other cavities in the coating. Upon cooling, the infiltrated CMAS composition solidifies and reduces the coating strain tolerance, thus initiating and propagating cracks that may cause delamination and spalling of the coating material.
In particular, molten dust reacts strongly with BSAS to form a low temperature eutectic and phases that are not stable in steam. Molten dust is less corrosive against rare earth silicates. Some rare earth silicates (e.g. those comprised of gadolinium, erbium, and yttrium) react with the molten dust to form highly refractory “apatite” phases. Others rare earth silicates allow CMAS penetration but do not suffer melt point suppression. All rare earth silicates, however, are mechanically weakened by their interaction with molten dust, such that subsequent erosion and impact events can more easily take off the coating.
A need exists, therefore, for coating compositions that are less susceptible to molten dust attack, and also less susceptible to subsequent gas erosion, particle erosion, and particle impact over the current state-of-the-art EBC materials.